What might Janie’s hair symbolize?

Janie's hair is a symbol of her strength and individuality. The prevailing social standards of the town dictate that it is unseemly for a young lady to wear her hair down. But Janie doesn't care; she'll wear her hair the way she wants to. In doing so, she is openly and defiantly asserting her individuality.
It's also notable that Janie's hair is described in almost phallic terms, illustrating how her strength as a woman transgresses established norms. For a woman to live life on her own terms is considered scandalous in this neck of the woods. Normally, it's only men who get to act this way. But Janie is different, and her assertiveness, as well as her rebellious free spirit, represent a serious threat to dominant social values and to Jody's masculinity.
Janie's hair is also straight, a characteristic associated with Caucasians. This notable feature of hers, a product of her mixed-race heritage, allows Janie to subvert not just traditional gender roles, but racial roles as well.